In the prior art, integrated circuit chips have been electrically coupled to lead frames by means of a plurality of wire bonds. These bonds are typically formed by wire bonding apparatus.
As the circuit density of integrated circuit chips has increased, the accurate positioning of a wire bonding head used to form wire bonds with respect to the various bonding pads of an integrated circuit chip has been more and more difficult. In U.S. Pat. No. 4,441,205-Berkin, which is assigned to the assignee of the present invention, a pattern recognition system is disclosed and claimed which alleviates the difficulties of positioning a wire bonding head with respect to the bonding pads of an integrated circuit chip. As chip density has continued to increase, however, a new problem has arisen.
As integrated circuit densities have continued to increase and the circuits have become more complex, the configuration of the lead frames which provide input and output to the chips has also become more and more complex. The lead frame beams to which wire bonds must be made have become increasingly smaller and more numerous and, therefore, more difficult to form wire bonds thereto. The decrease in lead frame beam size, however, is not the only problem. Another problem arises because the manufacturing tolerances of lead frames are such that the position of a given beam in a given lead frame configuration may vary from one lead frame to the next. The techniques employed in fabricating integrated circuit chips themselves are very precise and, therefore, the position of bonding pads on an integrated circuit chip do not vary significantly from one chip to the next. The techniques of forming lead frames, however, are relatively less precise and, therefore, the actual position of a given lead frame beam may vary from one lead frame to another. As chip densities have increased, therefore, the task of forming wire bonds to lead frame beams has grown more difficult because the target areas to which bonds are to be formed smaller than previously and the position of those target areas is variable.
Initially, the positioning of the wire bonding head with respect to a lead frame beam was done blindly. With blind bonding, the wire bonding apparatus simply assumed that the lead frame beam to which a wire bond was to be made was exactly where it was expected to be. Such an approach was suitable for relatively large sized lead frame beams. As lead frame beams decreased in size and chip densities increased however, such a technique was no longer applicable.
The next technique employed for the positioning of a wire bonding head with respect to a lead frame beam was manual. With this manual technique, the relative position of the lead frame beam was moved with respect to the wire bonding head by a human operator before the wire bond was formed. Such manual techniques, however, had very low throughput and were labor intensive.
In a third technique which has been employed, the position of the lead frame beam was adjusted with respect to the wire bonding head in accordance with the sensed position of the lead frame beam, the position being sensed by light reflected from the lead frame beam. With this third technique light from a light source was focused upon the lead frame. Light, reflected from the lead frame beam, was sensed by a light detector. The output from the light detector thus provided an indication of the lead frame beam position. The relative position of the lead frame was then varied with respect to the wire bonding head in response to the sensed position of the beam before the wire bond was formed.
At least two such reflected light techniques have been employed and both suffer from disadvantages, especially with certain types of integrated circuit lead packages. Lead packages fall into two categories. In the first category, after the lead frame is joined with respect to the integrated circuit chip by means of wire bonding, the chip and lead frame package are encapsulated in a thermoplastic material. Such packages are adaptable to techniques for beam lead positioning which employ reflected light because the reflectance of the lead frame beams in these packages is clearly distinguishable from that of the background. In a second category of integrated circuit packages, however, the lead frame is first encapsulated in a ceramic material and the integrated circuit chip is bonded to the ceramic encapsulated lead frame. Such packages are referred to as ceramic dual in line packages or cerdips. With cerdip packages, the reflectance of the integrated circuit beam is similar to the reflectance of the surface of the glass-like material found between the lead frame beams. Certain material located between the lead frame beams in such packages, sometimes referred to as interlead glass, has a reflectivity similar to that of the metal of the beams themselves. Because of this fact cerdip packages have been known to misinform those systems which use reflected light for sensing position, resulting in wire bonds being formed in the wrong location.
Since it is important for position sensing systems used with wire bonding apparatus to be suitable for use with both cerdip type integrated circuit packages which suffer from problems caused by the reflectance of interlead glass, as well as standard lead frames which do not include interlead glass, prior art position sensing systems have not been suitable.
In one reflected light position sensing system which has been sold by the assignee of the present invention, another difficulty, in addition to the reflectance problems caused by interlead glass, has been presented. In that system, each lead frame beam of a lead frame package was translated with respect to a light source and light detector until the position of that lead frame beam was precisely known. Thereafter, the position of the lead frame beam was adjusted with respect to a wire bonding head such that the wire bond would be formed at the appropriate location. Such a technique was found to result in undue intermittent motion of the lead frame with respect to the wire bonding apparatus. This intermittent motion imposed severe loads on the drive mechanism for the wire bonding head.
It would be desirable to provide an image processing system and method for a wire bonding apparatus which was capable of accurately positioning a lead frame with respect to a wire bonding head.
It would be further desirable to provide an image processing system and method for a wire bonding apparatus having the capability of reliably locating a large number of extremely small lead frame beams.
It would be still further desirable to provide an image processing system and method for a wire bonding apparatus having improved throughput over manual techniques.
It would be still further desirable to provide such an image processing system and method which, though employing reflected light, is capable of distinguishing between the reflectance caused by interlead glass of ceramic dual-in-line packages, and the lead frame beams themselves.
It would be still further desirable to provide such an image processing system and method which could accurately determine the position of a plurality of lead frame beams without the necessity of complex motions of the lead frame beams with respect to a wire bonding head so as to minimize inertial problems.
These and other objects of the present invention are achieved by a system and method for determining the position of leads on a semiconductor lead frame with respect to a wire bonding apparatus. In carrying out the method of the present invention, the lead frame is illuminated with light from a light source. Light reflected from the lead frame is sensed and digital data representing that reflected light is generated. The digital data so generated is stored in a memory. A control means defines a corridor in that memory, the corridor having a length and a width. A first signature of data within that corridor is obtained, the first signature being a widthwise summation of data at each point along the length of the corridor. First order derivatives of the first signature along the length of the corridor are determined and stored in a first array. The first array is scanned to determine those points along the corridor having first order derivatives typical of lead boundaries. Other first order derivatives are deleted from the array. The first array is scanned by the control means to determine regions between candidate lead boundaries, the positions of such regions being stored in a second array. The second array is scanned to determine whether the regions between candidate lead boundaries have a dimension which exceeds the minimum dimension of leads.
In accordance with an important aspect of the present invention, if the semiconductor lead frame under consideration is a ceramic dual-in-line package, the method of the present invention further includes the step of defining a series of second corridors orthogonal to the first corridor. These second corridors are located at points along the first corridor determined to be candidate lead boundaries. Second signatures are obtained in each of the second corridors and the second signatures are analyzed to distinguish leads from specular reflection of light from interlead material.